Gateway High School in Aurora, where the families of the missing are meeting following the Aurora shooting. Photograph: Jonathan Castner/AFP/Getty Images

The latest senseless rampage to shock America will never be forgotten. This country still remembers the 1999 Columbine school shootings, for example. Indeed, you wouldn't be alone if the murder spree, apparently carried out by PhD student James Holmes, triggered memories of other mass killings like Columbine.

Holmes' loner personality is very familiar for that of a mass shooter. The suspect also falls in line with the 95% of rampage killers since 1949 that were men.

But beyond these basic descriptions, how does Holmes fit into the history of rampage killers?

What I've done is gathered some basic statistics to try and determine how Holmes is like and unlike past mass murderers. I looked at 165 mass killers from 1949 to the present day. It's definitely possible that I missed some murders though I believe I have a solid baseline.

I have tried to contain my analysis to murders with multiple victims, at least one of whom died, and that were not the result of domestic strife, gang violence, robbery, terrorism or serial killings. That description of rampage killings and much of my data in the years to 1999 comes from a 2000 New York Times study.

Age

If your memory is like mine, you might have thought that most mass killings were carried out by young people, such as James Holmes. That's actually a misconception. The plurality of all murders, 36.6%, are committed by men between the ages of 18-24, like the 24 year-old Holmes, but most rampage killings are not.

Mass murder rates by age. Photograph: Guardian

Only 17.6% of mass killers are between the ages of 18 and 24 years old. Mass murderers do make up a larger share among the population at large in the 18 to 24 and 25- to 34-year-old demographic, but the plurality, 35.8% of rampage killers, are actually 35- to 49-year-olds. This percentage is well above the 20.5% of the entire population that is 35-49 years-old. Few mass murderers are above the age of 50.

Overall, the median age for rampage killers is 33 years old compared to the 27 years old median for all murders, which are both lower than the median age United States population of 37.

Place of shooting

Part of the reason you might think rampage killers are relatively young is because nightmares like Columbine stick with you. The truth is that 18.8% of rampage killings were committed by non-faculty at a school. While some of these heinous acts were committed by grad students, 50% were completed by those 17 or younger and 75% by 25 years old and younger. Many of these students believe they were bullied or were outcasts.

A higher 29.7% took place at person's place or former place of work. The median age for these work-place rampage killings was 39, which is right around the now 41-year-old median age of the American worker. Some workplace killings, but by no means a majority, were part of the infamous chain of unrelated postman killings. Many were workers who thought they had been unjustly passed up for a promotion or fired.

More than half (51.5%) of mass murders were at neither a workplace or a school. Holmes fits into this group, but again he is younger than most killers. The median age for rampage killers that don't commit their crimes at school or work is 35 years old. Many of these murderers, like Tucson shooter Jared Loughner, had a clinically diagnosed mental illness such as schizophrenia.

Murder/suicide

While we might have visions of mass murderers committing suicide before authorities can capture them, most do not. Of the 165 killings in this dataset, only 65, or 39.4%, took their own lives in the immediate aftermath of the rampage. This number has not gone up or down significantly with time. Age does not seem to impact a shooter's willingness to take their own life either.

Just under half (46.9%) of rampage murders who kill at the workplace took their own lives compared with 36.2% of school or other shootings, but that difference is not statistically significant because of the relatively small sample size. Either way, Holmes' surrender to the police fits in the majority for both mass killing overall and where he is alleged to have carried out the shooting.

Year trend

One clear trend in the data is that these sort of rampage killings seem to, for whatever reason, occur at a greater frequency than ever before. There have been six mass killings in 2012 alone. This trend would make sense if the number of overall murders was also rising, but that's not happening.

The reason behind the spike in recent mass murders is a subject of much debate. Photograph: Guardian

It turns out that murders have mostly been on the decline since the early 1990s, despite a rising population. Rampage killings, on the other hand, reached their peak in the late 1990s when the overall number of murders were reaching their lowest point since 1970. In fact, 75% of mass killings since 1960 have taken place in the past 24 years.

What's the cause? That's a big topic of debate. Twelve years ago, the New York Times looked at the data and believed that the rise in killings in the mid-to-late 1990s was because of a relaxation of gun laws. Gun rights proponent and academic John Lott disagreed claiming that the Times simply did not include a lot of data prior to 1995. My own finding concurs that the Times missed data, but that the gap between murders and mass murders from the mid-to-late 1990s onward still exists.

A host of explanations could explain this gap from missing data to relaxed gun laws to luck and everything in-between. More research is needed.

Conclusion

There will be much discussion over James Holmes in the months ahead. The unspeakable crimes he is alleged to have committed was a rare occurrence, but tends to fit the mold of such killings. Holmes was a male who is slightly younger than most assailants, but not overly so. He did not go to school or work at the place of the shooting and surrendered to police.

The big question that will be asked in the weeks and months to come is why are mass shootings maintaining their levels while the overall murder rate is at its lowest level in decades?